Most large (Mw >
7.0) and great (Mw>8.0) underthrusting earthquakes
nucleate along a
shallow region of
unstable frictional stability on or near the subducting plate interface
termed the seismogenic zone. Traditionally, the seismogenic zone
has been defined as the partially or fully coupled portion of the plate
interface capable of generating large or great earthquakes, bordered
updip and downdip by largely aseismic zones where brittle failure
cannot initiate but may propagate (e.g., Scholtz, 1998; Hyndman et al.,
1997). In the absence of
one or more full seismic cycles along a subduction zone, characterizing
the seismic hazard of a region has been done using proxies (i.e.,
temperature isotherms, extent of geodetic locking, location of
background seismicity, prior damage extent). The concept of
velocity-dependent material properties, the increasing recognition of
slow and aseismic slip within subduction zones, and the
occurrence of tsunami earthquakes suggest that a complex interaction of
processes controls the rupture limits of large and great earthquakes.
High-precision earthquake relocations and 3D velocity modeling can lend
insight into how thermal, mechanical, compositional, hydrological, and
rupture processes interact within the subduction zone. Such studies
allow comparisons between seismicity patterns and velocity
perturbations that identify heterogeneities along the subduction
megathrust that potentially influence the rupture patterns of the great
earthquake sequences. Increased precision earthquake catalogs constrain
subduction zone geometry, providing necessary input parameters for
modeling main shock rupture, calculating moment, or modeling source
characteristics of both the main shock and aftershocks.
Comparisons of aftershock locations to prior regional seismicity using
high resolution hypocenters provide insight into seismogenic activity
leading up to the main shocks.

My
primary research topics encompass many aspects of seismotectonics and
allow me
to combine my broader interests in large-scale plate tectonics problems
with
seismology. I am particularly
interested in understanding the process of lithospheric recycling and
earthquake generation within subduction zones, the role of fluids and
fluid
pressure in seismic wave generation, and the driving mechanisms leading
to
variability in characteristic seismic behavior along fault zones. The
majority
of my research to date utilizes earthquake relocation, local earthquake
tomography, and waveform cross-correlation techniques. However, I do
not
consider my research to be limited to a particular set of tools, and I
strive
to expand my skills to incorporate other geophysical datasets. I
have recently been working with
broadband waveform modeling, Empirical Green's Function deconvolution,
and
Coulomb stress transfer modeling to study individual large magnitude
earthquakes.
Subduction
Zone ProcessesAftershock studies of the 2004 M
9.2 Sumatra-Andaman Islands and 2005 M 8.7 Nias earthquakesThe
seismic and tsunami hazard posed by great subduction zone earthquakes
has long been recognized, but in light of the Mw 9 Sumatra-Andaman
Islands earthquake, the resulting tsunami, infrastructure damage and
huge loss of life, it is clear that the hazard posed by such events is
generally underestimated. The December 26, 2004 Mw 9.0 earthquake is
one of the four largest in the instrumental record, comparable in many
respects to the 1960 Mw 9.5 Chilean and 1964 Mw 9.2 Alaska subduction
megathrust events. The main shock and aftershock sequence essentially
illuminated the entire Andaman micro-plate along the Andaman subduction
zone and back-arc spreading center, and the earthquake likely triggered
the March 28, 2005 Mw 8.7 earthquake along the adjacent Sunda
Trench.

The 2004 earthquake initiated along the Andaman subduction zone, north
of the last great Sumatra earthquake along the Sunda Trench in 1861.
During the Nias earthquake, a portion of the 1861 rupture subsequently
failed. The boundary between the 2004 and 2005 ruptures broadly
coincides with local trench rotation and the southern edge of the
Andaman microplate, which suggests structural control on fault
segmentation. Aftershock relocations of the 2004 and 2005 earthquakes
using the EHB method show little overlap, and the sharp boundary
between the series locates near the 2002 Mw 7.3 Northern Sumatra
earthquake. We posit that these features represent the southern extent
of the stable Andaman microplate, ~50-100 km northwest of what was
previously reported. I have additionally used broadband analyses to
show that the 2002 earthquake was a bilateral rupture. The
resulting rupture pattern for the 2002 event was used to model Coulomb
stress changes near the 2004 hypocenter to assess stress interactions
along adjacent fault segments [see DeShon
et al., 2005].

Combined, the 2004
and 2005
sequences have generated over 5000 teleseismically-recorded earthquakes
along 1700 km of the Andaman and Sunda subduction systems. The
seismogenic portions of the Andaman and Sunda Trenches lie within an
oceanic environment, which ultimately limits the amount of local
seismic data that will become available and emphasizes the need for
state-of-the-art analysis of the available teleseismic data. Developing
and applying new teleseismic techniques to provide earthquake locations
and velocity structure resolution at a scale comparable to that of
regional network studies will help advance our understanding of the
complex ruptures of the Sumatra great earthquakes. To date,
aftershocks through September 2005 have been relocated using EHB
methods (pictured below), and while errors remain on the order of 10s
of km, interesting
patterns in both the seismicity and focal mechanism solutions have been
revealed [Engdahl et al., 2007
in press].

Double seismic zone identification
using teleseismic data

Understanding
the seismicity and structure of subducting slabs has important
geodynamic implications for volcanic and mantle processes. One
interesting, but poorly understood, feature of some subduction zones is
the development of two distinct layers of seismicity within the oceanic
lithosphere at intermediate depths (~50-300 km). Such
observations are aptly termed Double Wadati-Benioff Zones or Double
Seismic Zones (DSZ) and have been attributed to petrologic reactions
that trigger earthquakes via dehydration embrittlement. Identification
of DSZ has typically been limited to areas where dense local networks
cover the subduction zone. We are developing a new technique that
strictly uses globally recorded teleseismic waveforms to more precisely
determine the location of subduction zone earthquakes. By
cross-correlating the depth phase relative timing (pP-P, sP-P, and
sS-S) we improve differential travel time precision and hence the
relative depths of hypocenters. The technique facilitates
identification of DSZ, and we are in the initial stages of a global
review of subduction zones. We are
currently focusing on Alaska (H. Zhang), Java (M. Brudzinski), northern
Chile (H. DeShon), and the Marianas Trench (H. DeShon), where either
teleseismic data hints at DBZs
or local data has shown the existance of a DBZ. We have found a significant variability between
subduction zones of the characteristic upper and lower plane stress
pattern, confirming that not all DSZ exhibit downdip compression
overlying downdip compression. The ability to identify DBZ away
from local networks will be a key step in determining the overall
prevalence and regularity of this feature on a global basis, which is
necessary to understand the conditions, both seismologic and
petrologic, in the evolution of subducting slabs in general. Principle Investigators: Cliff
Thurber and Bob Engdahl.

Most large (Mw >
7.0) and great (Mw>8.0) underthrusting earthquakes nucleate along a
shallow region of
unstable frictional stability on or near the subducting plate interface
termed the seismogenic zone. My dissertation, entitled Seismogenic zone structure along the
Middle America subduction zone, Costa Rica, explored spatial and
temporal variability in microseismicity within the seismogenic zone of
the Middle America Trench (MAT) offshore Costa Rica and investigated
processes controlling the loci of shallow (<100 km depth) subduction
zone seismicity. The research was a primary component of the
Costa Rica Seismogenic Zone Experiment, a series of
interdisciplinary
geodetic, seismic, and fluid flux experiments undertaken from September
1999-June 2001 to better understand subduction zone behavior near the
Osa (southern) and Nicoya (northern) Peninsulas of Costa Rica. We utilized a range of absolute and relative
earthquake location and local earthquake tomography techniques to
relocate microseismicity and detect seismic velocity variations (VP,
VS, & VP/VS) along the subduction thrust in this region. The
CRSEIZE seismological studies constrain the updip and downdip limits
and along-strike variability of microseismicity along Costa Rica.
This data is essential for comparison with geodetic measurements,
thermal modeling, and hydrologic modeling to better understand the
process controlling seismicity along subduction megathrusts. Combined,
the studies provide the highest resolution images of seismicity and
seismic velocity along the Middle America seismogenic zone to date and
lend further insight into the thermal, mechanical, hydrological, and
compositional interactions potentially responsible for controlling
shallow subduction zone seismicity.

Comparison of the Osa and Nicoya experiment-specific 1D velocity models
to previously reported country-wide 1D VP models, generally centered in
central Costa Rica, shows similar velocity gradients between 5 and 30
km depth with mantle velocities (VP>7.8 km/sec) constrained between
35-50 km depth in the CRSEIZE experiment models compared to the large
depth range (35-65 km) reported by other 1D velocity model studies [DeShon
et al., 2003; DeShon,
2004]. Differences in deep velocity structure between local
and regional-scale 1D velocity studies are attributed to the relative
location of the reference seismic station with respect to the
subducting Cocos Plate and continental Moho. A single station
receiver function computed for Global Seismic Network station JTS in
northwestern Costa Rica confirms the location of the continental Moho
between 33-40 km depth and indicates 15% serpentinization of the mantle
wedge near the Nicoya Peninsula [DeShon and Schwartz,
2004]. Hypocenter relocations across the Osa and
Nicoya regions indicate a deepening of the updip limit from south to
north, paralleling the steepening of the subducting Cocos Plate along
strike of the Middle America Trench.

Absolute and relative relocations of ~300 aftershocks of
the 1999 Quepos, Costa Rica, underthrusting earthquake are
used to analyzes seismogenic zone structure offshore central Costa Rica
during a period of increased seismicity rate. DeShon et al.
[2003] shows that subduction of
highly disrupted seafloor north of the Osa Peninsula has established a
set of conditions that presently limit the seismogenic zone to be
between 10-35 km below sea level, 30-95 km from the trench
axis. Aftershock locations define a plane dipping at
19° that marks the interface between the Cocos Plate and the Panama
Block. Relative event relocation produces a seismicity pattern
similar to that obtained using absolute locations, increasing
confidence in the geometry of the seismogenic zone. The aftershock
locations spatially correlate with the downdip extension of the oceanic
Quepos Plateau and reflect the structure of the mainshock rupture
asperity. This strengthens an earlier argument that the 1999 Quepos
earthquake ruptured specific bathymetric highs on the downgoing plate [Bilek et al., 2003] The areas of
mainshock moment release are marked by the star & box on figure to
the right.

Volcano
seismic networks typically have few stations and marginal coverage,
providing challenges for earthquake location in a complex,
three-dimensional setting. Routine catalog locations are
performed using analyst phase picks and an approximate, 1D velocity
model. To improve earthquake location precision we compute a
three-dimensional P-wave velocity model using double-difference
tomography combined with waveform cross-correlation techniques.
Waveforms recorded at volcanoes are often noisy and/or emergent. We use
waveform cross-correlation techniques to improve the pick accuracy of
catalog data. Differential travel times for well-constrained
events are used to simultaneously invert for hypocenter location and
P-wave velocity structure. The double-difference tomography method
provides significantly improved absolute and relative earthquake
locations. Synthetic models are used to assess the accuracy of the
resulting 3D models. In association with the Alaska Volcano
Observatory and Univ. Wisconsin Madison, we are simultaneously
computing relative earthquake locations and 3D velocity models for some
of the better monitored volcanoes along the Alaska Volcanic Arc,
including Spurr (J. Brown), Redoubt and Augustine (H. DeShon),
Shishaldin (N. Meyer), and
Pavlof and Great Sitkin (J. Pesicek). This work takes advantage
of advances in earthquake
location and inversion techniques (tomoDD) as well as waveform
cross-correlation (BCSEIS). Principle Investigators: Cliff
Thurber, John
Powers, and Stephanie Prejean.

Arenal Volcano,
Costa Rica

From May 1995
through February 1999, UCSC and OVSICORI-UNA
collected time-continuous seismic and geodetic data at Arenal Volcano,
Costa Rica. Arenal, a young
active
stratovolcano located in northern Costa Rica, began erupting in 1968
after
a 450 year period of quiescence and is currently in a Strombolian
phase,
producing small summit explosions, intermittent lava flows, and
occasional
pyroclastic flows.Seismic
signals
recorded at Arenal include long period (1-3 Hz) transients related to
summit explosions and almost continuous, and frequently harmonic,
volcanic tremor. Daily statistics calculated for explosions and
tremor provide a near continuous record of seismic activity at Arenal
and show a decrease in summit activity from 1995-99, possibly
reflecting a change within the volcano's magmatic system.
My current work with the Arenal dataset includes exploring
statistical
relationships within the explosion and tremor datasets to better
understand
interactions between the physical mechanism(s) producing these seismic
signals
within the volcanic system. Frequency-amplitude analysis
for
summit explosions indicate explosion occurrence may follow power-law
scaling,
analogous to the Gutenberg-Richter relation for earthquake occurrence,
suggesting
a scale-invariant process produces summit explosions. Previous
work
on Arenal's volcanic tremor also suggests a scale-invariant, possibly
chaotic,
process produces tremor as well [Julian,
2000]. We are exploring temporal
relationships between volcanic tremor amplitude and frequencies and
summit
explosion occurrence to lend further insight into the degassing
processes
active within the volcano during periods of high and low summit
activity.

Planetary
Geology

Diana Chasma, Venus

My interest in
planetary geology and geodynamics stems from research
conducted with Vicki Hansen and Duncan Young while in undergrad at
Southern Methodist Univ. My
senior thesis project involved detailed geologic and structural
analysis of the Diana-Dali Chasmata region on Venus,part of the USGS Geologic Investigation
Series Atlas of Venus [Hansen and
DeShon, 2002]. We predominately used synthetic aperture
radar (SAR) data collected by the Magellan spacecraft to map
corona, chasmata, fracture patterns, lava flows, craters, etc.
Using
cross-cutting relationships, we were able to piece together a geologic
historyfor plains formation in this
region
that suggests plains formation on Venus occurs through discrete
volcanic
processes working at local and regional, rather than global, scales [DeShon
et al., 2000]. I have let my research in planetary
geology
fall by the wayside, but I maintain an strong interest in the subject.